1. Field of the Invention
The present invention relates to communication systems having nodes that arbitrate for control of the bus and that transfer data on the bus. More specifically, the present invention relates to a node that has a first bus configuration for bidirectional arbitration and a second bus configuration for unidirectional data transmission.
2. Description of Related Art
Every communication system having a number of devices (nodes) that compete for a limited resource (a communication bus) must first arbitrate to determine which node will next use the bus. After one node wins, then the communication system must allow the winning node to use the bus without interference from the other nodes. Thus, a bus architecture for a communication system must be designed to facilitate both arbitration and data transfer.
One common bus architecture includes a single physical bus, for example a cable, that is directly connected to each node. Any node coupled to the single bus can transmit a signal which is received by other nodes; i.e. the single bus is designed for communication between any of the nodes. Inter-node communication is particularly useful because any node must be able to place a signal on the bus during arbitration and all other nodes must receive that signal. Arbitration methods to determine which node will next use the single bus are well known, and may include any method such as collision detection, collision avoidance, and token passing.
After arbitration is complete, the data transfer phase begins in which only one node will have control of the bus. All others are either receiving or not listening. Because only one node is transmitting during the data transfer phase it is not necessary that the bus be bidirectional. In the single bus example, transmitted data propagates unidirectionally from the winning node to each of the receiving nodes.
A single bus has a number of disadvantages which become very apparent at higher rates of communication, particularly during the data transfer phase. In general, length and construction can adversely affect the bandwidth of a bus, thereby limiting the maximum speed at which data can be transferred.
One particular problem with a single bus is the number of taps made into the cable to connect the devices. Each tap introduces an impedance discontinuity, causing reflections and losses which adversely affect electrical performance. The more taps, the more performance is degraded. Impedance discontinuities can be avoided by the use of splitters; however splitters are expensive, and they cause one-half of the power to be diverted in each of two directions. Therefore even a few splitters will greatly reduce power and substantially degrade performance.
If a single bus is implemented in a silicon chip, interconnection technologies currently available can greatly reduce the problem with impedance discontinuities. However, for devices that may be distributed over many meters, a single silicon bus is simply not feasible. In summary, operational speed during arbitration and data transfer using a single physical bus is limited by the construction of the bus.
It would be an advantage to provide a communication system with nodes having a plurality of ports that can be connected by point-to-point links, thereby providing a significant speed advantage compared with traditional multi-access buses. It would be an advantage if the nodes have a bus architecture that provides a first bus configuration for arbitration in which the bus can be treated as a single logical bus, and a second bus configuration for high speed unidirectional data transfer without the bandwidth limitations of a single bus. It would be a further advantage if such a system could be implemented with a plurality of nodes connected in a tree configuration, each node having a short silicon bus for high speed data transfer, and if the data could be resynchronized and retimed in each node during data transfer for higher bandwidth, thus transferring data at high rates.